Because the typical standard of consent is that in order to do something to someone, you should have informed consent. If you cannot obtain that, then you do not do the thing. Something that does not exist cannot give informed consent, therefore you should not do the thing.

That’s just how evolution works- something that already exists and is driven to stay alive is more likely to pass on its genetics than something that is not driven to stay alive. This fact has nothing to do with the philosophy of consenting to exist in the first place.

Edit: missed your first question. Something that does not exist cannot desire.

Something that no one has discussed in this highly enlightened conversation here is the issue of consent. A person cannot consent to being born. Full stop. I don’t know of a way around that besides ignoring it.